Printing plates may be imaged on a plate-making machine and then transferred to a printing press. Once on the printing press, the images from the printing plates are transferred to paper or other suitable substrates. It is important that images printed using a printing press be properly aligned with the substrate on which they are printed. Obtaining such alignment typically involves:                carefully aligning a reference edge of a printing plate with pins or other features on the plate making machine;        detecting one reference point on an orthogonal edge of the printing plate (i.e. orthogonal to the reference edge) at a known distance from the reference pins;        imaging the printing plate; and        using the reference edge and the orthogonal edge reference point to align the printing plate on a drum of the printing press.        
One common technique of aligning the printing plate on the drum of a printing press involves using the reference edge and the orthogonal edge reference point to align the printing plate on a punching machine and punching registration holes in the printing plate. The printing plate may then be aligned on the drum of the printing press with registration pins that project through the registration holes.
Traditionally mechanical alignment pins have been used to align the plate to be imaged to the drum of a platesetter. This is not a flexible arrangement. The pins have to be mounted in predetermined positions. There are also reliability challenges in consistently and accurately loading the plate into contact with the pins. It is also difficult to define sets of pins that allow a wide range of plate formats to be imaged whilst not interfering with one another.
There is therefore a need for an alignment mechanism not based on mechanical locating pins. A number of these have been proposed. Examples are disclosed in and in U.S. Pat. No. 6,318,262 (Wolber et al.) in both of which edge detection sensors are employed in the load path to an imaging drum upon which a printing plate is imaged. U.S. Pat. No. 4,881,086 (Misawa) also describes a laser recorder with sheet edge detection based on the principle of the difference in reflectivity between that of the sheet and that of the drum on which it is carried. EP 1 081 458 A2 (Elior et al.) teaches the use of an apparatus to determine a skew angle of the plate mounted on a plate support surface. U.S. Pat. No. 4,876,456 (Isono et al.) describes using photosensors having light emitting elements and light receiving elements disposed in a path for carrying a photosensitive film. EP 1 081 458 A2 describes an apparatus for detecting a plate edge using a light beam and detector.
In U.S. Pat. No. 6,815,702 (Kiermeier et al.) a method and apparatus are disclosed to locate an edge of an imageable plate mounted on a drum or other support surface. A light source and light sensor are used to measure the difference in reflectivity between the plate and the support surface. The drum or support surface contains at least one groove to increase the difference in reflectivity between the plate and the support surface. The groove may also contain an anti-reflecting layer to further increase the difference in reflectivity. The groove may also have a geometric shape that causes incident light to be directed away from the light sensor. U.S. Pat. No. 6,815,702 describes that, with the groove parallel to the drum axis, an edge of the plate is “generally perpendicular to the groove” when the plate is “properly mounted.” It also explains that the groove cannot be parallel to and positioned under the edge of the plate, as this makes precise detection of the plate “unreliable, and near impossible” by the method of the patent.
A further important aspect of the entire plate alignment process is the method of loading of the plate onto the imaging drum. While there is some description in the prior art of systems for correcting the placement of a plate on a drum, it is generally more effective to get the plate loaded as close to perfectly aligned as possible during the initial loading step. In the case of the method described in U.S. patent application Ser. No. 11/693,007, the fully loaded printing plate needs to be protruding over the slot and aligned as closely as possible with the edge of the slot.
U.S. Pat. No. 6,604,465 (Tice et al.) describes the loading of a printing plate onto an external drum while rotating the drum in a first direction. No mention is made of rotating the drum in another direction while loading the plate. While the patent does disclose a method for aligning of the printing plate without requiring any holes to be punched in the printing plate, alignment of the printing plate is in fact done using pins on the drum. The printing plate is then imaged while the drum is rotated in the first, or in a second, opposite direction. Finally, the printing plate is unloaded from the drum while rotating the drum in the second direction.
U.S. Pat. Nos. 6,260,482 and 6,189,452 (both to Halup et al.) respectively describe a method and apparatus for loading and unloading plates to external drum devices based on movable clamps. The system is characterized by clamps, ideally in pairs of which the members are circumferentially disposed with respect to each other, that are movable over the surface, preferably along circumferential tracks, enabling the attachment of multiple plates, end-to-end and/or side-by-side. To mount a printing plate on the drum, a first clamp of each relevant pair is first opened and then engaged to the leading edge of the printing plate, which is fed from a suitably positioned loading mechanism, and then releasing the clamps to grip the leading edge of the printing plate. The drum is then rotated in a first direction to pull the plate and wrap it around the drum. Then the other clamp of each pair is opened and the drum is rotated in a second opposite direction, while the clamp remains stationary, until the trailing edge of the plate is engaged by the clamp, whereupon the clamp is released, thus gripping the trailing edge by slidable clamps. After normal imaging operation, the plate is demounted in the same general order, by first releasing the first clamp of each pair (which grips the leading edge of the plate) and moving it away from the plate in the second direction of the drum, thus freeing that edge, then rotating the drum in the first direction, thus pushing the plate onto a suitably position unloading bin, and finally releasing the second clamp of the pair, thus freeing the plate. No mention is made of moving the drum in different directions in order to correctly position the plate on the drum.
U.S. Pat. No. 5,992,325 (Schumann et al.) describes a method for automatically detecting the trailing edge of a printing plate. The detecting can be the determination within a trailing edge clamp of either the location of the edge, or the determination of the presence of the trailing edge. To this end, a sensor is employed. This patent also discloses a method for loading the printing plate. The method starts, after release of the trailing edge of a previous plate, with the ejection of the previous printing plate, which is achieved by rotating the drum in a first direction to push the plate by its leading edge. The leading edge of that plate is then unclamped. The drum is then rotated in a second, opposite direction by a very small amount, enough to clear the leading edge of the previous plate. The drum is then rotated in the first direction again to receive the leading edge of a new plate into the same clamp from which the leading edge of the earlier plate has been ejected. The presence and or location of this leading edge is determined by the sensor. The printing plate is then clamped by its leading edge. The next step comprises rotating the drum in the second direction in order to wrap the printing plate on the drum. Suitable steps are taken to tauten the printing plate on the drum and to secure the trailing edge. While the patent describes small rotations of the drum to load and release the printing plate, it does not address the matter of alignment of the printing plate or its exact positioning relative to any possible slot in the drum.
In commonly-assigned U.S. patent application Ser. No. 11/204,223 an edge detection system is described, based on using a digital camera to image the edges of a printing plate perpendicular to the sub-scan direction. Based on the information so obtained, the image data is then adjusted to compensate for any misalignment between the plate and the drum on which it is loaded. In commonly-assigned U.S. patent application Ser. No. 11/693,007 an edge detection system is described, based on using a digital camera to image the leading edge of a printing plate. The system employs a slot in the cylindrical surface of an imaging drum, the slot having a radially recessed surface that has diffusely reflective surfaces and substantially non-reflective surfaces. The system allows the leading edge of a printing plate protruding over the slot to be located through the leading edge clamps by illumination with a suitable illumination source and imaging with a digital camera.
Commonly-assigned U.S. patent application Ser. No. 11/693,007, incorporated herein in full, requires a newly loaded printing plate to be protruding over the slot and aligned as closely as possible with the edge of the slot in the drum described in that patent application. The prior art does not describe how this is to be achieved.